1. Field of the Invention
This invention relates to a liquid interface diffusion (LID) bonded composition and a method of preparing such a composition. More particularly, this invention relates to the use of metallic alloys for joining nickel-based superalloys in a LID bonding process. The composition and method of this invention are useful where high strength, heat resistant materials are required, such as in aircraft and aerospace-related structures.
2. Background Information
Brazing and diffusion bonding methods for the joining of nickel-alloy structures are known to those skilled in the art. However, few of these are well-suited to bonding nickel-alloy honeycomb sandwich panel structures. A limited number of techniques exist for diffusion bonding nickel-alloys, such as Hot Isostatic Pressing (HIPing) of plasma sprayed coatings (U.S. Pat. No. 4,592,120), boron-enriched nickel-alloy foils (U.S. Pat. No. 4,700,881), or other techniques involving the sputter-deposition of boron onto the mating surfaces of a joint (U.S. Pat. No. 5,836,075). Another technique exists for repairing cavities in e.g. turbine engine components using metallic powder pastes containing boron and silicon (U.S. Pat. No. 5,806,751). However, in the case of components fabricated from nickel-based superalloy honeycomb core and face sheets, joining techniques have previously been limited to brazing, and no liquid interface diffusion process has been available.
Certain oxidation resistant nickel-based superalloys such as INCONEL 617 with relatively high aluminum-contents are also difficult to braze due to poor wettability by most brazing alloys. This disadvantage renders these alloys unsuitable for brazing and they are therefore of limited use in the form of honeycomb sandwich panel applications where oxidation resistance is required. This problem could be avoided if a LID-type process could be used to join nickel-alloys. In LID-bonding, the high degree of fusion does not require good wettability of a braze alloy across the joint surfaces.
Many brazing alloys used for joining nickel-based superalloys contain boron to lower the melting point of the nickel-rich braze alloy to form a joint. Although boron is very effective in this capacity, its rate of diffusion at elevated temperature is extremely rapid. This is particularly the case for diffusion along grain boundaries in the parent material, as noted in e.g. U.S. Pat. No. 4,700,881. This phenomenon leads to the potential problem of boron diffusing away from the joint interface (in the solid state) before the brazing alloy has been brought up to its melting temperature. If the assembly to be joined cannot be raised to the brazing temperature fast enough, there is a risk that excessively rapid diffusion of boron will prevent adequate melting of the braze alloy and no bonding will occur. An excessive sensitivity to process thermal cycles can make brazing difficult for large honeycomb sandwich structures where thermal equilibrium cannot be established quickly. This problem would be less acute in the case of LID-bonding provided that some other alloying element was substituted for boron to achieve a sufficient depression of the melting point of the parent material.
A further disadvantage of brazing processes is the significant compositional heterogeneity across the joint interface. This is due to the general lack of fusion between the braze alloy and the parent material. When used in corrosive environments (such as some fuel combustion atmospheres) this phenomenon has the potential of increasing the susceptibility of the bonded structure to micro-galvanic corrosion. This disadvantage could be eliminated with the significantly greater degree of fusion associated with a LID-bond if a LID-bonding system could be derived for joining nickel-based superalloys.
U.S. Pat. No. 4,059,217 is directed to LID bonding nickel- and cobalt-based superalloys. However, the filler alloys specified in this patent also include boron (to permit melting as described above) and therefore are prone to the same disadvantages as boron-containing braze filler alloys. An additional disadvantage of the technique described in U.S. Pat. No. 4,059,217 relates to the plating method used to apply the LID-bonding alloy, which does not always guarantee a uniform thickness of deposit. This problem can be overcome in the foil application method described in the current invention.
U.S. Pat. No. 4,126,449 is directed to amorphous zirconium-titanium alloys with iron, cobalt, nickel or copper as the third element. The invention relates only to the alloy compositions and not to any specific application of the alloys.
U.S. Pat. No. 4,676,843 is directed to a process for joining component workpieces made of a superalloy employing a diffusion bonding process. The diffusion bonding process requires the powdered form of the parent alloy, i.e. a powder having either the same or a very similar composition to the parent material of the mating surfaces. As a consequence, no transient liquid phase would be formed as a result of diffusion between the filler alloy and the parent material as is the case in LID bonding. In the method described in U.S. Pat. No. 4,676,843, heat and pressure are used to consolidate the powder into a solid in a relatively large gap of (initially) 1 to 2 mm (0.04 to 0.08xe2x80x3) between the mating surfaces. For this process to be successful, the mating surfaces have to be either large in area or have grooves cut into them, in order to contain the powder. This method would not be suitable for joining core to face sheets in honeycomb panel applications. In these fabrications, the joint interface area is relatively small, due to the thin gage of the honeycomb foil (typically 0.002 to 0.003xe2x80x3). It would clearly be impractical to attempt to cut grooves in the edges of the honeycomb cell walls to contain the powdered form of the filler alloy for the method of U.S. Pat. No. 4,676,843.
U.S. Pat. No. 4,681,251 is directed to a method of joining nickel-alloy articles such as gas turbine engine components. The method uses an aluminum film which is sputtered onto the mating surfaces of the workpieces in a physical vapor deposition (PVD) process under vacuum. The method of this invention is well suited to small articles such as actively-cooled turbine blades of the type which are exposed to highest operating temperatures in aircraft jet engines, as shown in FIG. 1 of U.S. Pat. No. 4,681,251. However, the method used to deposit the bonding alloy (in this case aluminum) is not well suited for joining large components such as nickel-alloy honeycomb sandwich panels and ducts. This is because the sputtering process (where atoms or ions of the filler alloy are produced from a point source) cannot easily deposit a coating of uniform thickness over a large area of typically several sq. ft.
A method and composition prepared by a LID bonding method are disclosed in U.S. patent application Ser. No. 09/736,774. The method and composition of this U.S. patent application are directed to a titanium honeycomb core and titanium facing sheets and require therebetween a foil that comprises copper, nickel, zirconium and titanium. The foil of the filler alloy in this patent application has a composition specifically selected to produce a transient liquid phase with the particular composition of the titanium-alloy of the mating surfaces of the parent material. The transient liquid phase is based on a multi-component eutectic alloy formed between the filler alloy and the parent alloy. As such, the filler alloy composition of U.S. patent application Ser. No. 09/736,774 cannot be used to join nickel-alloys, which are the subject of the current invention.
The present invention uses a foil interlayer of an alloy for use in applications involving the joining of nickel honeycomb materials. The present invention allows for a greater degree of fusion across the joint interface than would be possible with conventional brazing technology. This provides greater mechanical integrity of the bond and significantly greater peel strength for honeycomb structures made from nickel-based superalloys. In turn, this requires fewer repair pins to be used in those honeycomb structures. Repair pins are often used in large numbers on conventionally brazed honeycomb structures made from nickel-based superalloys in efforts to guarantee the mechanical integrity of the brazed joints.
It is one object of the present invention to provide a liquid interface diffusion bonded composition in which at least one metal honeycomb core is bonded to a metal facing sheet, wherein a foil interlayer comprising nickel, zirconium, and at least one additional metal selected from the group consisting of titanium, niobium, and chromium is used to join the mating surfaces of the honeycomb core and facing sheet by being rendered liquid at the bonding temperature and thereby forming a liquid interface for effecting diffusion bonding of the core and facing sheet. It is another object of this invention to provide a method of preparing such a composition. The LID process of the present invention enables the bonding of such metal components, including the bonding of slightly mismatched mating surfaces of such components, and is less critically dependant on furnace heating rates than is the case with conventional boron-containing braze alloys. This reduces the incidence of in-process dis-bonds and further reduces the need for costly re-work. The composition and method of this invention are useful in applications where high strength, heat resistant materials are required, such as in aircraft and aerospace-related structures.
The liquid interface diffusion bonded composition of this invention comprises a metal honeycomb core and a metal facing sheet bonded thereto, wherein the composition is prepared by a method comprising:
(a) providing a nickel-alloy honeycomb core having a mating surface and a nickel-alloy facing sheet having a mating surface;
(b) placing together the honeycomb core mating surface and the facing sheet mating surface, and providing therebetween a metal foil comprising nickel, zirconium, and at least one additional metal selected from the group consisting of titanium, niobium, and chromium;
(c) subjecting the mating surfaces and metal foil therebetween to sufficient positive pressure to maintain position and alignment for joining; and
(d) heating the mating surfaces and metal foil therebetween in a protective atmosphere for at least 2 hours to a temperature sufficient to cause melting between the metal foil and mating surfaces of the facing sheet and honeycomb core.
In one preferred embodiment of this invention, the liquid interface diffusion bonded composition of this invention comprises a metal honeycomb core and a metal facing sheet bonded thereto, wherein the composition is prepared by a method comprising:
(a) providing a nickel-alloy honeycomb core having a mating surface and a nickel-alloy facing sheet having a mating surface;
(b) placing together the honeycomb core mating surface and the facing sheet mating surface, and providing therebetween a metal foil comprising nickel, zirconium, and at least one additional metal selected from the group consisting of titanium and niobium;
(c) subjecting the mating surfaces and metal foil therebetween to sufficient positive pressure to maintain position and alignment for joining; and
(d) heating the mating surfaces and metal foil therebetween in a protective atmosphere to a temperature in the range of about 1840 to about 1900 degrees F. for at least 2 hours, preferably 3 hours, to cause melting between the metal foil and mating surfaces of the facing sheet and honeycomb core.
In another preferred embodiment of this invention, the liquid interface diffusion bonded composition of this invention comprises a metal honeycomb core and a metal facing sheet bonded thereto, wherein the composition is prepared by a method comprising:
(a) providing a nickel-alloy honeycomb core having a mating surface and a nickel-alloy facing sheet having a mating surface;
(b) placing together the honeycomb core mating surface and the facing sheet mating surface, and providing therebetween a metal foil comprising nickel, zirconium, and chromium;
(c) subjecting the mating surfaces and metal foil therebetween to sufficient positive pressure to maintain position and alignment for joining; and
(d) heating the mating surfaces and metal foil therebetween in a protective atmosphere to a temperature in the range of about 2500 to about 2700 degrees F. for at least 2 hours, preferably 3 hours, to cause melting between the metal foil and mating surfaces of the facing sheet and honeycomb core.
Preferably, the metal foil of the present invention does not comprise copper and does not comprise boron and does not comprise silicon. In one preferred embodiment, the metal foil comprises about 10 to about 14 wt. % zirconium and about 8 to about 12 wt. % titanium, the remainder being nickel. In another preferred embodiment, the metal foil comprises about 10 to about 14 wt. % zirconium and about 20 to about 28 wt. % niobium, the remainder being nickel. In another preferred embodiment, the metal foil comprises about 10 to about 14 wt. % zirconium and about 45 to about 50 wt. % chromium, the remainder being nickel.
In preferred embodiments of the composition and method of this invention, the facing sheet is a nickel-alloy facing sheet, and the metal foil is formed by a rapid solidification process or a melt spinning process.
The composition and method of this invention are useful in applications where high strength, heat resistant materials are required, such as in aircraft and aerospace-related structures.